The present invention relates to a power supply unit comprising a multiplicity of cells such as lithium secondary cells, nickel hydrogen cells, lead seal cells, electric double layer capacitors and fuel cells connected in series parallel, and a distributed power supply system and an electric vehicle including them.
In the case where a plurality of cells are connected in series, the variations of capacitance, initial voltage and temperature from one cell to another causes a different voltage for a different cell, thereby making it difficult for all the series-connected cells to share the voltage across the circuit uniformly.
Especially in the case where the lithium secondary cells or the electric double layer capacitors employing an organic solvent as an electrolytic solution are connected in series, voltage variations causes an overcharge or an overdischarge, often resulting in a rupture or a fire, or at least an overcharge or an overdischarge, which poses the problem of an extremely shortened service life of the cells.
In order to prevent the overcharge or overdischarge, the charge/discharge operation may be performed with a pre-set protective level. In charge mode, however, the charge operation stops when the voltage across a high-voltage cell has reached the protective level. As a result, the remaining low-voltage cells fail to be fully charged before the end of the charge operation.
In similar fashion, the discharge operation stops at the time point when the voltage across a low-voltage cell has reached a protective level. As a result, the remaining high-voltage cells cannot be fully discharged before the end of the discharge operation.
In the series connection of cells, therefore, the charge/discharge time becomes shorter than in the case where each cell is charged/discharged independently.
In a conventional battery charging apparatus intended to solve this problem, the charge current supplied through a bypass is changed by a current changing means progressively according as the voltage across the cells being charged approaches a set value thereby to set the cells into a uniform state. Examples are illustrated in U.S. Pat. No. 5,557,189 and a corresponding Japanese Patent No. JP-A-7-230829. FIG. 12 is a diagram showing such a battery charging apparatus. In FIG. 12, reference numerals 1101a to 1101c designate cells, numerals 1102a to 1102c voltage detection means, numeral 1103 set voltage application means, numerals 1104a to 1104c comparison control means, and numerals 1105a to 1105c current changing means. The circuit for the cell 1101a is so configured that the voltage detection means 1102a, the comparison control means 1104a and the current changing means 1105a are connected in parallel to each other, and the set voltage application means 1103 applies a set voltage indicating the setting of a voltage value of the cell 1101a. 
The present voltage value of the cell 1101a is detected by the voltage detection means 1102a, and compared in the comparison control means 1104a with the set value of the voltage applied by the set voltage application means 1103a. According as the present cell voltage approaches the set voltage value, the charge current flowing in the current changing means is increased progressively. Specifically, the charge current to the cell 1101a is controlled progressively downward. In this way, an overcharge is prevented.
The fact about the cell 1101a described above equally applies to the cell 1101b and the cell 1101c. In other words, the voltage detection means 1102b, the comparison control means 1104b and the current changing means 1105b for the cell 1101b, and the voltage detection means 1102c, the comparison control means 1104c and the current changing means 1105c for the cell 1101c, work exactly the same manner as the corresponding means, respectively, of the cell 1101a. 
Another example of the prior art is disclosed in JP-A-2000-78768. This is intended to correct the variations caused at the time of charging the lithium ion secondary cell and to prevent the trouble such as overcharge for an improved service life. Specifically, a negative electrolytic solution circulation pump and a positive electrolytic solution circulation pump are used for correcting the variations of the charge/discharge operation. Still another example of the prior art is disclosed in JP-A-2000-511398. This is a system for equalizing the cells and is a combination of energy storage elements that can be switched. Specifically, the charge is shifted between batteries each including a plurality of cells connected in series. The charge is pulled out of a particular battery of a higher voltage and transferred to another battery of a lower voltage.
In the conventional battery charging apparatus, a cell voltage at the time of charging is compared with a set value, and with the approach of the cell voltage to the set voltage value, the charge current is progressively diverted to the current changing means in parallel to the cells thereby to assure uniform conditions of the cells.
According to the prior art, however, the amount of current that can be diverted is greatly limited by the heat generated in the current changing means. Thus, the effect of obviating the voltage variations among the cells is reduced. The current changing means having a large thermal capacitance through which a large current can flow, on the other hand, is large in size and the system becomes bulky. Also, an electrical circuit other than the cells is required and increases the cost. The method of circulating the electrolytic solution, on the other hand, requires a pump. Also, a battery equalizer including a switch circuit for moving the charge by switching and a control circuit for the switch circuit is required.
The present invention has been developed in view of the problems described above, and the object thereof is to provide an inexpensive, compact power supply unit which can correct the voltage variations among cells connected in series.
According to this invention, there is provided a power supply unit comprising a first cell group and a second cell group connected in parallel to the first cell group, in which the electrolytic solution of the second cell group can be electrolyzed or the generated gas can be recombined. A plurality of the parallel-connected pairs are connected in series to each other and also to a charger/discharger. The charger/discharger is adapted to charge the cells at appropriate timing to a voltage at which the electrolytic solution of the second cells is electrolyzed or a voltage at which the generated gas is recombined. As a result, a plurality of parallel-connected pairs including the cells of the first cell group and the second cell group are equalized at a voltage at which the electrolytic solution of the cells of the second cell group is electrolyzed or a voltage at which the generated gas is recombined.
In the parallel-connected pair of the first cell group and the second cell group according to the invention, the first cell group and the second cell group are connected in parallel through a current limiter. The current limiter limits the current flowing between the first cell group and the second cell group, and prevents the overcurrent of the first cell group or the second cell group, thereby making it possible to protect the power supply unit at the time of a fault.
A plurality of series circuits including the parallel-connected pairs of the first cell group and the second cell group are connected in parallel. As a result, the capacitance, the output and the service life of the power supply unit can be variably increased.
In this invention, the withstanding voltage of the cells of the first cell group is set to a level higher than the withstanding voltage of the cells of the second cell group. Specifically, the electrolytic solution of the cells of the second cell group is electrolyzed or the gas is generated and recombined within the operating voltage range of the first cell group. In this way, the cells of the second cell group and the cells of the first cell group are equalized at a voltage at which the electrolytic solution is electrolyzed or the generated gas is recombined, as the case may be, in the cells of the second cell group.
According to another aspect of the invention, there is provided a power supply unit, wherein at least selected one of the first cell group and the second cell group includes a plurality of cells connected in series. This circuit includes at least an intermediate terminal for each appropriate number of the series-connected cells, in addition to a main positive terminal and a main negative terminal. The first cell group and the second cell group can be connected in parallel through the intermediate terminal and the main terminals.
According to still another aspect of the invention, there is provided a power supply unit, wherein the first cell group and the second cell group share at least one component element. As a result, the number of parts and the cost are reduced. The component element shared is preferably the electrolytic solution.
Also, carbon fiber or carbon nanotube is added to the electrodes of at least selected one of the first cell group and the second cell group. Especially in the batteries with the electrodes thereof extended or contracted at the time of charge/discharge operation, the resulting stress is relaxed by the carbon fiber or the carbon nanotube, as the case may be.
According to yet another aspect of the invention, there is provided a power supply unit, wherein the parallel-connected pairs of the first cell group and the second cell group are connected in parallel to a cell management circuit. As a result, the equalization of the voltage of the parallel-connected pairs can be enhanced and the conditions thereof can be detected.
According to a further aspect of the invention, there is provided a distributed power supply system in which a cell power supply unit including the first cell group and the second cell group is connected in parallel to a second similar power supply unit, the system comprising a charger for performing the charge operation in such a manner that in the case where the second power supply unit is deficient of power, the first power supply unit assists in supplying power, while in the case where the second power supply unit generates extra power, the charge operation is continued by a charger, using the extra power, up to a voltage at which the electrolytic solution of the cells of the second cell group of the first cell power supply unit is electrolyzed or a voltage at which the generated gas is recombined.
According a still further aspect of the invention, there is provided an electric vehicle comprising a motor-generator for driving the vehicle and regenerating power, and a cell power supply unit connected to the motor-generator, wherein the cell power supply unit includes a first cell group and a second cell group connected in parallel to the first cell group, the power supply unit further comprising a charger capable of charging the cells of the second cell group up to a voltage where the electrolytic solution of the cells of the second cell group of the cell power supply unit is electrolyzed or a voltage at which the generated gas is recombined.
The invention is applicable to various cells including the lithium secondary cells, the nickel-hydrogen cells, the lead seal cells and the electric double layer capacitors or the fuel cells connected in series parallel.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.